Check how many primary instances have conflicting tags. The
algorithm to compute this is to sum the count of all tags, then
subtract the size of the tag map (since each tag has at least one,
non-conflicting instance); this is equivalent to summing the
values in the tag map minus one.

Adds a secondary instance (doubly extended version). The first parameter
tells addSecExEx to ignore disks completly. There is only one legitimate
use case for this, and this is failing over a DRBD instance where the primary
node is offline (and hence will become the secondary afterwards).

Transform a Nodes + Instances into a NodeGraph with all reboot exclusions.
This includes edges between nodes that are the primary nodes of instances
that have the same secondary node. Nodes not in the node list will not be
part of the graph, but they are still considered for the edges arising from
two instances having the same secondary node.
Return Nothing if the node list is empty.